UPDATE TO MATH -
perky mcjuggs (god, i hope you're single and wanting to get a coffee, ya'know, or somethin') and uniter pointed out an omission in the math - update is below rest of post
yep, only 19 pleadged delegates will mathematically end the opportunity for hillary to win the pledged delegate race.
once obama scores 19 magic number delegates, the only way hillary can win is by attracting a strong majority of superdelegates.
stay with me, it will all make sense when you look at the math.
in baseball there is an old saying, "you're going to win one-third of your games. you're going to lose one-third of your games. its what you do with the middle third that matters."
proportional delegate awards in primary states means that each candidate will get at least one-third of the delegates in the remaining states. that has been the case in all the previous primaries (excepting home states, nebraska, dc, and idaho).
the magic number, for those of you who don't know, is used in sports to track the viability of a particular team in a division race. basically, it is the number of games a leading team needs to win to "clinch" the division. after a leading team "clinches" the second place team can still win all of their remaining games and not be able to take first place.
let's do the math
from pennsylvania through south dakota there are 567 pledged delegates to be determined.
each candidate will win one-third of them, just by being on the ballot. as i said, when there are only 2 candidates on the ballot and it only takes 15% of the vote in a district to get a delegate, every candidate is going to go home with some delegates. 33% seems to be the minimum for any particular state. so each candidate will win 189 delegates, just for being listed on the ballot.
the question becomes - who wins the middle 189 votes? the trouble for hillary is that she starts that race with a deficit of 171 (in pledged delegates, using the nyt list). she needs to win 172 of the 189 possible delegates (91%) to win the pledged delegate race by 1.
or, to state the same facts from obama's perspective. both candidates are going to get one-third of the delegates in the remaining races. if obama gets 19 delegates out of the middle 189 delegates (10%) he statistically eliminates hillary from wining the pledged delegate race.
so, all obama needs to mathematically eliminate hillary from the pledged delegate vote is 71 (out of 158) pennsylvania delegates. if he scores 45% of the penn delegates the pledged delegate race is over. mathematically.
now, we all know the superdelegates can overturn the popular choice. but that would be all that's left to hillary's strategy.
19 delegates. and that number decreases for each delegate he wins until the last primary. one-third plus 19.
UPDATE
perky mcjuggs and uniter point out that i omitted counting some delegates. so, really the magic number is 10. see their post below for the details - no need to double that effort.